1. Field of the Invention
The invention relates generally to the field of medical imaging. More particularly, the invention relates to fast magnetic resonance imaging of different chemical species, such as in imaging of water, fat, and silicone implants.
2. Discussion of the Related Art
Magnetic resonance imaging (MRI) can be used to generate chemical shift specific images because protons in different chemical species may have different resonance frequencies. For in vivo tissues, the two most dominant chemical species are water and fat, whose resonance frequencies are separated by approximately 3.5 ppm (parts per million), or 220 Hz at 1.5-Tesla field strength. Multi-point Dixon (MPD) can also be used to separate water and fat images by encoding the phase difference between water and fat signals into images with different echo shifts and by subsequent post-processing. One advantage of the MPD techniques is that field inhomogeneity effects on the images can be completely removed in post processing. However, the MPD technique requires the acquisition of multiple images, which leads to increased total scan time.
An estimated 2 million women in the United States alone have silicone breast implants. Because ruptures or leakages may pose significant health risks, accurate radiological evaluation of the additional chemical species, such as silicone breast implants is of high importance. Presently, the most widely used diagnostic modalities for silicone breast implant evaluation are x-ray mammography and ultrasound sonography. Unfortunately, the radiological findings using these techniques are generally not conclusive.
Magnetic resonance imaging (MRI) has proven useful in the diagnosis of ruptures or leakage of silicone gel-filled implants and, in general, is more sensitive than competing modalities. One of the primary reasons for this high sensitivity is due to the fact that MR imaging facilitates the acquisition of silicone-specific images in the breast, permitting unequivocal determination of intra- or extracapsular ruptures of silicone-based prostheses.
There are generally two types of methods for generating silicone-specific images using MR. The first is the frequency selective method. One possible implementation is to use a frequency-selective excitation or refocusing pulse centered on the resonance frequency of silicone in conjunction with other techniques to suppress the water and lipid signals. Since resonance frequency of fat and silicone are too close to separate robustly based on resonance frequency, fat suppression is often achieved via the use of short-tau inversion recovery pulse sequence, which takes advantage of the characteristically short longitudinal relaxation time of fat.
Another potential implementation is to use a combination of two preparatory pulse sequences before the acquisition of silicone images. The first sequence is the short-tau inversion recovery (STIR), which is used to suppress the fat signals based on its short longitudinal relaxation time. The second sequence is the chemical saturation (ChemSat) sequence, which is used to suppress the water signals based on its chemical shift. While these techniques may provide clinically useful images, the image quality that is achieved could be sub-optimal or inconsistent under different scan configurations because of the techniques' intrinsic sensitivity to the magnetic field inhomogeneity.
Another method that produces silicone-specific images is the phase-selective method. The Dixon technique, originally proposed to generate separate water and fat-only images, belongs to this category. In order to be adapted to the silicone implant imaging, previous investigators have assumed that the frequency separation between silicone and water resonance is a multiple of the silicone and fat resonance frequency difference. Such an assumption was necessary because of the presence of the three distinct chemical species (water, fat, and silicone) in contrast to the water/fat imaging where only two chemical species (water and fat) are involved. One obvious drawback of this approach is that the above assumption is not realistic.
Furthermore, even if the assumption is valid, the method can only be used with the traditional symmetric Dixon sampling where the two chemical species under consideration are set to be strictly either in-phase or 180 degrees out-of-phase. Using asymmetric sampling, which has been shown to offer increased time efficiency and processing reliability, would put water and fat out of phase, and therefore render the Dixon approach completely inapplicable to silicone imaging. Consequently, this approach is limited in time-efficiency and reliability. This method also requires relatively long acquisition times, often leading to reduced slice coverage, compromised imaging parameters or exacerbated motion artifacts.
Initial comparisons between the frequency and phase selective methods in the literature demonstrate the degradation of diagnostic quality associated with the limitations of the phase-selective approach. Recently, more sophisticated approaches relying on spectral modeling have been introduced. While these models make no assumptions about the frequency spectrum, they do require extensive post-processing involving inversion of potentially unstable matrices. As for the Dixon technique, the spectral modeling technique also requires long acquisition time, and scan parameters that can be used are therefore limited due to patient comfort and motion.